I'm loading mine with the 70-grain Hornady GMX seated close to the lands, and it feeds pretty well. But am guessing it would feed even better if it was a .22-250 Improved, since the 6.5 Creedmoor is essentially a 6.5/.250 Improved, and my RAR Predator 6.5 Creed feeds really well, with either the magazine it came with, or the one from the .22-250. But the tapered case of the .22-250 can indeed cause feeding problems even in other rifles with magazines designed for straighter-sided cases like the .243/.308.
Out of curiosity just checked the four websites where I order most of my reloading supplies. Turns out all had 6mm Remington brass, but only two had .223 WSSM's. Hornady's making both now, however, so real shortages of either shouldn't be common.
One of the reasons the WSSM's didn't do so well is they required special Browning or Winchester bolt actions, while the .22-250 and .22-6mm will work in any common action. The .22-6mm I had was built on a long 700 action, so had LOTS of room for bullet seating, yet fed perfectly without any tweaking of the standard magazine.
